E. coli is the most widely applied microorganism. Especially in the field of molecular biology and genetic engineering, E. coli variants are essential host cells used in labs for mass-producing different kinds of DNAs or proteins. The technology of delivering DNA molecules in the surrounding medium into host cells is called transformation, and the host cells after preliminary treatment to be more permeable to DNA molecules are called competent cells.
Hence, producing competent cells and transforming competent cells is very important in view of recent developments of genetic engineering. The technology described above can be retraced to Mandel, M. and Higa, A. (J. Mol. Biol. 53:159-162), who published a chemical transformation method using CaCl2. After 30-year-improvement, the time needed for transformation is still 1.5-3.0 hours because the host cells are injured by the chemical treatment. The injured host cells require a recovery step. In the recovery step, the host cells are cultured in a nutrient medium to allow the injured host cells to recover their physiological function and drug resistance. Then the host cells are plated on a selective medium to screen for transformed host cells. Otherwise, the transformation efficiency would decrease by several times. Recently, a fast transformation method called electroporation can deliver DNA molecules into the E. coli host cells by transient current. However, the host cells after the transient current treatment still need one hour of recovery to obtain a higher transformation efficiency (Dower et. al., 1988 Nucleic Acids Res. 16: 6127-6145). In 1988, Golub E. I. (Nucleic Acids Res. 16: 1641) published a method of one-minute transformation. Although a recovery step is performed, the transformation efficiency is only 104-105 colonies/μg plasmid DNA.
A fast method of transformation (U.S. Pat. No. 6,864,088), in which the long and time-consuming recovery step of competent cells is omitted from the conventional transformation method without reducing its transformation efficiency, has been developed by Yeastern Biotech Co., Ltd in recent years. The fast method comprises the following steps: mixing plasmid DNA and the competent cells in a tube, heat-shock treatment of the mixture, and then plating the mixture on a selective medium of lower temperature with a plating tool of lower temperature. The recovery step of adding non-selective medium (e.g., SOC, LB . . . ) is omitted. Therefore the conventional multi-step transforming procedure taking 1.5˜3.0 hours is shortened to a few minutes and can be completed within one single tube, without reducing its transformation efficiency.
Although said method of transforming competent cells is better than prior arts, a temperature-control apparatus for heat-shock treatment is still indispensable. Further, the cells may not be heated evenly when placed in a tube. Hence, it will save a large amount of time, expenses and efforts if an innovative method of transformation is developed in which the time-consuming recovery step of the transformed cells is omitted, a temperature-control apparatus is not required, cells may not be heated unevenly and the transformation efficiency may not be reduced.